This invention relates to a solid state image sensor, and more particularly, to a high-speed solid state image sensor which is used for image read out devices.
Information processing systems, for example computers, have advanced rapidly, and their processing abilities are superior to those of humans in a limited area. However, devices which input data to information processing systems have not advanced commensurately. Efforts have been made to develop more advanced image readers in the field of facsimile, because of the industry's urgent need for practical devices. One of the technical objects has been to achieve high resolution image sensing as well as to reduce the size of the device. Another object has been to read out color images.
Furthermore, in the field of copiers attempts have been made to form the picture elements with dot type or heat-sensitive transfer systems instead of the conventional electrophotographic process. In this case, higher speed and resolution have been required of the input device than are needed with an ordinary facsimile apparatus.
Solid state scanning systems exist which are capable of satisfying the above requirements. For example, there is one which comprises photodiode arrays acting as photoelectric converters, and MOS switches for scanning the arrays. Another system combines, in a single type of semiconductive element, the functions of both image resolution and information storage. These solid state image pickup elements are highly integrated so the size of one chip is much smaller than that of a document. Therefore, an optical mechanism is required for reducing the size of the image. For example, as noted in U.S Pat. No. 4,217,435 at col. 1, 1.66 through col. 2, 1.12, currently-available CCD or photodiode arrays are constructed as integrated circuits and are therefore very small in size. In order to read a large document using a single, small CCD array, one must employ an optical system which reduces the image to a size comparable to the integrated array. Such an optical system is necessarily large. On the other hand, using a 1:1 optical system (such as self-focus lenses) is also disadvantageous because the size of the photosensor array which would be required with a 1:1 optical system is extremely large. Moreover, to use separate semiconductor switches (rather than integrated arrays) for the photosensor and associated apparatus would be much too expensive and would produce a device too large for practical purposes.
The problem of covering a large area using a series of small photosensor arrays has been treated in U.S. Pat. No. 4,005,285. There, a beam splitter is used to divide an image into two images for projection onto two rows of photosensor arrays, the arrays being staggered so that they are, in effect, combined into a single line of photosensitive elements. In this device, problems of positioning the photosensor arrays are a serious drawback. First, they must be positioned directly under the image projected by the beam splitter, which is constructed to produce two image lines a fixed distance apart. The beamsplitter cannot be adjusted to move one of the image lines if the distance between the photosensor arrays is incorrect; the arrays themselves must be moved. This is difficult when one considers that the width of a single line of photosensitve elements is in the region of 10.sup.-5 m. Second, the photosensitive element arrays must be positioned longitudinally so that the optical effect is one continuous line of photosensors (see FIG. 1B of the '285 patent). In other words, the effective gap between the last element in the array in one row and the first element in the next array in the other row must be the same as the gap between two adjacent photosensitive elements, which is considerably less than 10.sup.-5 m. If the gap is too great, information will be lost. If there is no gap, but rather some overlap, blurring or additional undesired characters will be present in the reproduced document. Furthermore, the necessity that the lens system be aligned properly with the beam splitter presents an additional obtacle to accurate image sensing with the device of the '285 patent.
Image sensors have been developed in which the lens system is omitted, and in which the document (which is the object) and the sensor or image directly correspond by a 1:1 ratio. This type of image sensor has no lens system, so the device can be compact. But in this type, the quality of the image data read by the sensor becomes extremely deteriorated, if the distance between the document and sensor is not maintained to close tolerances, for example, 10 um or so. Furthermore, in this type of device, the response characteristic of the sensor material is weak and high speed read-out is not possible.